When biocatalysts are used on an industrial scale, they are usually used in an immobilized form with the objectives of preventing elution of impurities from the biocatalyst, improving separability of the biocatalyst from the reaction product, improving applicability of the biocatalyst to repeated use, increasing enzymatic stability of the biocatalyst, and carrying out continuous operation of the production steps.
Immobilization of biocatalysts is effected by a carrier binding method, a cross-linking method, an entrapping immobilization method and the like (cf. T. Hattori and C. Frusaka, J. Biochem., vol. 48, pp. 831 (1960)), of which the entrapping immobilization method is most advantageous in that leakage of biocatalysts is small, and a decrease in the biocatalyst activity caused by the immobilization process is also small, because the biocatalyst and its carrier are not linked to each other. Furthermore, the method can be applied to the immobilization of a large variety of biocatalysts.
Examples of known carriers for use in the entrapping immobilization of biocatalysts include synthetic, high polymers such as polyacrylamide, polyvinyl alcohol, polyurethane, collagen, a photosetting resin and the like, and natural, high polymers such as carrageenan, alginic acid, agarose, starch, gelatin and the like (cf. U.S. Pat. No. 4,526,867). In general, in comparison with the natural, high polymers, the synthetic, high polymers are high in strength, excellent in durability and resistant to biodegradation. Of these, polyacrylamide is used most frequently because it is industrially inexpensive, has high polymer strength and causes less inactivation of biocatalysts at the time of polymerization (cf. U.S. Pat. No. 4,421,855 and I. Chibata, T. Tosa and T. Sato, Appl. Microbiol., vol. 27, pp. 878 (1974)).
In addition, a process has been proposed in which acrylamide, a cationic ethylenic unsaturated monomer, and a water-soluble cross-linking monomer are subjected to copolymerization in order to reduce degree of swelling of a polyacrylamide base immobilized biocatalyst obtained by the entrapping immobilization method and to reduce inactivation of the catalyst at the time of the reaction (cf. JP-B-58-35078; the term "JP-B" as used herein means an "examined Japanese patent publication").
However, such polyacrylamide base immobilized biocatalysts, obtained by the entrapping immobilization method, are generally stored by soaking in an aqueous solution such as a buffer solution or the like, since their activities are apt to decrease when exposed to air oxidation or drying. When the storage is continued at room temperature for a prolonged period of time (for example, more than 1 month), the storing solution and the biocatalyst start to putrefy which causes generation of offensive odors from, and turbidity in, the storing solution and causes a decrease in the activity to such a level that it cannot be used as a catalyst. As a consequence, their storage is effected generally by putting them in a refrigerator or adding an antiseptic agent to the storing solution.
However, the cold storage method requires a huge cost for facilities, utilities and the like when biocatalysts are used in an industrially large quantity, and the other method, in which an antiseptic agent is added to the storing solution, causes permeation of the antiseptic agent itself into the immobilized biocatalyst, thereby adversely effecting the quality of products for the practical use.
It is important to overcome these problems especially in the case of the industrial application of immobilized biocatalysts.